The invention relates to a for pickling high-grade steel, preferably high-grade steel hot strip, the high-grade steel being subjected to pickling with neutral electrolyte and a subsequent treatment with mixed acid, nitric acid or a mixture of iron fluoride and hydrofluoric acid.
In the high-grade steel manufacturing industry processes for pickling high-grade steel according to the neutral electrolyte process have become more and more generally adopted in recent years. As is, for example, described in AT-PS 252685, the material is for this purpose pickled in aqueous solutions of neutral alkali salts of mineral acids and subsequently in aqueous solutions of mineral acids, preferably sulfuric acid, both treatment steps being performed electrolytically, i.e. by applying an electric current. As neutral electrolyte an aqueous sodium sulfate solution is preferably employed, and the current densities are in the region of 5 and 15 Amp/dm.sup.2. Higher current densities are possible in principle but are not applied because of the higher voltage required due to the poor conductivity of high-grade steel. As is known from AT-PS 387406, the subsequent after-treatment in the acid, in this case preferably mixed acid, can be carried out currentless. The expression mixed acid denotes preferably a mixture of nitric acid and hydrofluoric acid. Pickling with applied currents in mixed acid is not generally practiced because of problems with the electrode material which would have to be current and acid proof.
Despite the satisfactory effectiveness of these known processes with regard to the pickling effect, it was found to be disadvantageous that these processes require a relatively long treatment period. For example, the pickling time in neutral electrolyte and mixed acid for an approximately 3 mm thick high-grade steel hot strip takes about 120 sec., for 6 mm thick high-grade hot strip about 240 sec. and for ferritic hot strip of the quality AISI 430 even up to 270 sec., so that, with a view to a more rapid and more economical production, a demand remained for a shorter pickling process.
Japanese patent application no 55-50468 describes a purely chemical process for pickling chromiferrous high-grade steel. In this case pickling first takes place without current in hydrochloric acid or sulfuric acid and a subsequent after-treatment is performed with Fe(NO.sub.3).sub.3, FE.sub.2 (SO.sub.4).sub.3 or (NH.sub.4).sub.2 S.sub.2 O.sub.8 -solution in order to attain a particular degree of whiteness. Scale removal and the attainment of a smooth, white surface is in this context achieved without current and only by the oxidizing after-treatment with salts of trivalent iron, respectively with persulfates. This process cited can, however, only be carried out in ferritic high-grade steel because, although austenitic high-grade steels contain chromium also, the scale is nevertheless composed in an entirely different manner because of the additional alloy element nickel and cannot be removed by this described pickling method alone.
Accordingly a need still exists for a process of the type mentioned in the opening paragraph according to which, while maintaining or even increasing the satisfactory pickling effect achieved up to date, a shortening of the treatment period can be achieved at the same time.